Various implantable medical devices for repairing or reinforcing cardiac and vascular structures have been developed in recent years. Some of these devices can be implanted inside a particular vascular structure through so-called interventional, or endovascular, techniques. Interventional techniques involve surgically accessing the vascular system through a conveniently located artery or vein and introducing distal portions of a medical device assembly into the vascular system through the arterial or venous access point. Once the medical device assembly is introduced into the vascular system, it is threaded through the vasculature to an implantation site while proximal portions of the assembly having manually operated control means remain outside the body of the implant recipient. The medical device component of the assembly is then deposited at the implantation site and the remainder of the distal portion of the medical device assembly removed from the vascular system through the access point.
Exemplary interventional medical device assemblies include a catheter. The catheter can be used to precisely position the medical device at an implantation site as well as participate in deployment of the medical device at the implantation site. Some catheters have guidewires running their length to aid in positioning and deployment of the medical device. As an alternative to the guidewire, a catheter can have a moveable inner sleeve running inside the length of the catheter. The inner sleeve is used to push an implantable medical device out of, or simply away from, the distal end of the catheter. Handles, knobs, or other manually operated control means are attached to the opposite end of the guidewire or inner sleeve in the assembly.
Some implantable medical devices, such as stents and stent-grafts, often require reconfiguration from an initial compacted form to an expanded cylindrical configuration as the device is deployed at an implantation site. These devices can expand on their own by virtue of the design and composition of their structural elements or through the use of an inflatable balloon placed inside the devices.
Interventional medical devices are maintained in a compacted configuration in a variety of ways. Some devices are maintained in a compacted configuration by simply confining the compacted devices inside a catheter, or similar tool. Other devices are held in a compacted configuration with a removable line of material threaded through structural elements of the devices. These devices are free to expand when the drawstring is withdrawn from the structural elements of the devices. Yet other devices are placed inside a removable or breachable sheath following compaction. In these devices, the sheath is usually removed or breached by pulling on a drawstring, or similar control line, attached to the sheath.
U.S. Pat. No. 6,352,561, issued to Leopold et al., teaches the use of a control line made of a polytetrafluoroethylene suture material to initially close a restraining member around a self-expandable medical device. The sheath-like restraining member is closed around the self-expanding medical device by bringing opposite sides of a planar restraining member together in the form of a tube and stitching the control line along the length of the restraining member to form a seam. In preferred embodiments, the control line is stitched in a chain-stitch pattern that permits the control line to become unstitched from the restraining member when pulled upon by a practitioner. As the control line becomes unstitched from the restraining member, the self-expanding medical device begins to expand and displace the restraining member from around the device. When porous polytetrafluoroethylene materials are used for the control line, force exerted on the control line by the self-expanding medical device following release of the first few chain-stitches can cause the control line to become unstitched along portions of the restraining member without pulling on the control line any further. While the lubriciousness and biocompatibility of porous polytetrafluoroethylene control lines are desirable in this application, it would be advantageous to decrease the tendency of a porous polytetrafluoroethylene control line to become unstitched from a restraining member by the forces of an expanding medical device. A control line having more tensile strength, higher modulus and structural rigidity, and/or less compressibility than a porous polytetrafluoroethylene control line would cause the control line to become released from the restraining member only when the control line is pulled by a practitioner. This would provide a practitioner with more control over the release of the control line from the restraining member. Indeed, such a control line would provide a practitioner with tactile feedback through the control line of the release of each chain-stitch. Such a control line could be provided with a lubricious, biocompatible, fluoropolymer covering and maintain its desired stitch retaining properties.
It would also be advantageous to use a control line with as small a diameter as practical in order to reduce the size and increase the flexibility of the catheter component of an implant assembly.
An implantable medical device assembly that would achieve these advantages would utilize a control line having a small diameter core made of a high tensile strength non-fluoropolymer material surrounded by a fluoropolymer material.